Conventionally, in a rubber composition using a diene rubber as a rubber component, a zinc oxide is used as a vulcanization accelerator aid for aiding the acceleration of the vulcanization of the rubber composition. The zinc oxide has a primary particle size of ordinarily 1 μm or less, but when compounding together with other compounding agents by using an internal mixer or open roll, such as Banbury mixer, kneader and the like, since its aggregability is strong, it is difficult to obtain excellent dispersibility in the rubber component, and aggregates of tens of μm are frequently seen. Such aggregates of zinc oxide become fracture nuclei, and thus become the reason for deterioration of reinforcement performance (breaking strength), wear resistance, rupture strength, etc. of the rubber composition.
Regarding this, in order to efficiently disperse the zinc oxide in the rubber component, suggested is a method for previously kneading merely a rubber component containing a natural rubber and/or a diene synthetic rubber, a carbon black, and a zinc oxide, to thereby prepare a masterbatch (see PTL 1 in the following), etc. According to this method, by dispersing the zinc oxide at nanolevel, it is possible to obtain a rubber masterbatch having improved reinforcement performance (breaking strength), wear resistance and rupture strength, and a rubber composition using the same.